"Know, Harmis, that every nation on earth is but appearance and illusion...What can it matter to the masters of Chaos, if a world such as yours were here, or gone, or never here at all?"
- —Ystareth to Harmis Detz[1a]
Ystareth among a unit of Plaguebearers.[2a]
Ystareth is a katharos, a kind of Daemon more commonly known as a Daemon Prince of Nurgle, who was once a Human who entered voluntarily into the service of Nurgle, the Lord of Decay, to become a katharos, a champion of plague and pestilence. The motive behind this obscene act was revenge -- Prince Faramond of the realm of Khypris in the Border Princes had been indirectly responsible for Ystareth's brother's death, and in a fit of anger, the estranged sibling had made a pact with the Chaos Gods for the power needed to exact a terrible and bloody revenge.[1a]
Warped by bitter and angry feelings more than any effects of worship, what was once a Human form soon became twisted and disease-ridden. The newly-initiated Champion of Nurgle's body became bloated in the image of Nurgle himself, and the stench of decay and fetid odor of death smothered the last remnants of normality.[1a]
Harmis Detz face Ystareth[2c]
Ystareth is the true name -- given by Nurgle himself -- of this Champion of Chaos, who by sheer force of will rose to join the heady ranks of the Daemons almost immediately as a Daemon Prince, rather than suffering the ritual gathering of a warband in an attempt to gain favor with the Chaos God of pestilence and plague.[1a]
Ystareth conceived a plan of such proportions that it would bring the decaying realm of Nurgle to the Border Princes in all its putrefying glory. Everything Ystareth needs to exact revenge on the claret-and-gold livery of Faramond has been provided.[1a]
Ystareth appears in the physical form of a Great Unclean One. However, Ystareth has just a fraction of a true Greater Daemon's abilities. Although eventually destined to become a Greater Daemon, Ystareth still has some way to go before he will be able to claim such a level of power.[1a]
Ystareth can assume an illusionary appearance at will. He will maintain it under all circumstances except when spellcasting and will mask both his true foul appearance and even more foul stench so that he may move amongst the unsuspecting people of the Old World.[1a]
The mortal body of Ystareth was ultimately slain by the future Prince of Khypris, Harmis Detz, with a precise blow to his corrupt heart, sending the Daemon Prince's soul reeling back to the Realm of Chaos.[2b]